1. Field of the Invention
This invention relates to regulating transient responses of an output signal of electric generators.
2. Description of Related Art
When a load is suddenly decreased or removed from an output of an electric generator, the output voltage typically increases during a transient response. This increase in output voltage during the transient response is due primarily to two factors. The first factor is a delayed response of an electric generator controller that regulates an exciter current, which, by flowing through an exciter winding, provides an exciter magnetic field. An amount of energy coupled by transformer action is influenced by the exciter current alone during the transient response, given that a mechanical rotating force of the generator is constant during this period. A decrease in the exciter current decreases the amount of energy coupled by transformer action. Since the exciter current does not adjust instantly after the load is decreased, the amount of energy coupled by transformer action does not adjust instantly, and energy output of the generator remains constant initially. However, the energy going to the load is decreased, and consequently the energy output tends to increase the output voltage.
The second factor is due to a quantity of stored magnetic energy that is in the exciter winding of the electric generator immediately prior to the decrease or the removal of the load. In the ideal case, if the electric generator controller responded immediately to the removal of the load by reducing an exciter current which reduces the energy coupled to the output by transformer action, there is still the quantity of stored magnetic energy in the exciter winding. The quantity of stored magnetic energy is transferred by transformer action to an armature winding, where it again increases the output voltage of the generator.
Conventional electric generators, for example a brushed generator, have the exciter winding on a rotor. During the transient response, the conventional electric generators dissipate the quantity of stored magnetic energy in the exciter winding in a resistive impedance. The quantity of stored magnetic energy of the exciter winding is diverted to the resistive impedance when an output voltage is detected as having an over-voltage situation.
However, there are situations when it is difficult to employ the resistive impedance to discharge the energy in the exciter winding. For example, when the electric generator is a brushless generator. In this situation, an exciter circuit consists of an exciter field winding on a stator, an exciter armature winding on the rotor and a generator field winding on the rotor. The generator field winding, in this case, has a quantity of stored magnetic energy that must be dissipated when the sudden change or removal of the load occurs. Since the generator field winding is on the rotor, which does not have any electrical connections to the stator, it is difficult to employ electrical circuitry for the purposes of dissipating the quantity of stored magnetic energy in the generator field winding.
Conventional brushless generators as disclosed in U.S. Pat. No. 6,628,104 by Yao, for example, provide an impedance circuit that selectively and temporarily absorbs excitation field current in the free-wheeling path of the excitation field winding to reduce voltage overshoot of the generator upon occurrence of an operating transition, such as a transition from high load to low load. In one implementation, the impedance circuit is an RC circuit, a by-pass switch is provided across the RC circuit. When excitation current in the free-wheeling path is not to be absorbed by the RC circuit, the by-pass switch is ON, thereby providing a low-impedance path for the excitation current. A by-pass driver controls the by-pass switch to change the by-pass switch from ON to OFF based on one or more detection signals, e.g., indicating a load transition or power-up, thereby introducing the impedance circuit into the free-wheeling path to effect decay of the excitation current from the generator. This solution has the disadvantage that the energy stored in the generator field winding is not absorbed by the impedance circuit, and therefore the energy in the generator field winding can cause damaging over-voltage conditions on the output of the generator.
In another situation, when the load is suddenly increased on the output of the electric generator, the output voltage typically decreases in the transient response. This decrease is primarily due to the delayed response of the electric generator controller. The increased output energy requirement is not initially provided for by the amount of energy coupled to the output by transformer action, which is primarily influenced by the exciter current. The increased load tends to sink charge from the output capacitance at a rate greater than the amount of charge sourced to the output capacitance by transformer action, and consequently the output voltage drops. Clearly, the electric generator by Yao does not offer a solution to this problem.
To solve these problems a novel method and apparatus are required that prevents the energy stored in the generator field winding from causing an over-voltage condition on the output when the load is decreased, and prevents a decrease in output voltage when the load is increased.